The present invention relates to a method for connecting a wheel, such as a turbine wheel or a compressor wheel to a shaft of a turbine wheel rotor, particularly the turbine wheel of an exhaust-gas turbocharger for motor vehicles. The present invention also relates to a turbine wheel rotor, that includes a steel shaft and a cast wheel including a casting alloy fixedly connected to an end of the shaft.
Currently used turbine wheels are mostly based on Ni-based alloys. In isolated cases, turbine wheels made of TiAl have also been tested and used. According to the prior art, turbine wheels are first manufactured by precision casting or comparable methods and subsequently connected to the shaft in one or more operations. This is usually done by brazing or welding processes. Unlike the turbine wheel, the shaft is conventionally manufactured from steel. The connection must withstand very high mechanical loads, especially during acceleration processes.
At present, single-part bearing housings are used, the shaft being guided therethrough with the fixedly connected turbine wheel and, on the other side, being connected to the compressor wheel by means of a press-fit or screw connection.
The compressor wheels are preferably manufactured from aluminum alloys. This is usually done by precision casting. However, insufficient strength has resulted in that compressor wheels are sometimes also milled from the solid, which is much more cost-intensive. Currently, new approaches attempt to deal with the strength problems of compressor wheels by using titanium alloys.
In mass production, the conventionally used nickel-based turbine wheels are connected to the shaft using friction welding techniques. In the joining technique steel shaft TiAl wheel, usually methods are used in which the shaft is connected via an intermediate piece composed of austenitic stainless steel, of a heat-resistant steel, or of a superalloy based on Ni, Co, or Fe.
Intermediated pieces made of two interconnected cylinder sections are used as well. To connect the intermediate pieces to the shaft and to the wheel, both friction welding techniques and brazing methods are used.
A method for making an interconnection between a turbine rotor made of an intermetallic Ti—Al alloy and a steel component is known from European Patent Publication EP 0368642. The interconnection is accomplished by friction welding using an intermediate piece which is composed, for example, of an austenitic steel. In one embodiment, the intermediate piece was already connected to the Ti—Al alloy part by insert casting.
Japanese Patent Publication JP 02173322 describes an integrally formed Ti—Al turbine rotor composed of a wheel and a shaft.
Apart from single-part models, multi-part turbine rotors have the disadvantage of having to ensure a suitable connection of the individual parts.